Method and device for influencing combustion processes involving combustibles

ABSTRACT

A method and device are for influencing combustion processes, and include an electrical device. The electrical device engages with the flame front so that electrical field thus produced, only penetrates areas of the flame front in which a stabilizing and harmful-substance-reducing effect is produced. The electrodes of the burner are arranged outside the region of the flame in the associated device.

[0001] The invention relates to a method for influencing combustion operations with fuels, in which electrical means are used to guide and/or alter a flame at a burner. In addition, the invention also relates to a device for carrying out the method using stabilizing, pollutant-reducing means to influence the flame during the combustion operation.

[0002] The advantageous influences which electric fields can have on combustion flames have fundamentally long been known. According to the publications

[0003]Industrial and Engineering Chemistry 43 (1951), pages 2726 to 2731,

[0004] 12th Annual energy-sources technology conf. (1989), pages 25 to 31 and

[0005]AIAA Journal 23 (1985), pages 1452 to 1454 the effects of the electric field consist in an improvement to the flame stability. According to

[0006]Combust. Flame 78 (1989), pages 357 to 364 and

[0007]Combust. Flame 119 (1999), pages 356 to 366 the carbon emissions are reduced, and according to

[0008]Fossil Fuel Combustion, ASME 1991, pages 71 to 75 and

[0009]Fluid Dynamics 30 (1995), pages 166 to 174 the emission of gaseous pollutants is reduced.

[0010] It is also known from Combust. Flame 55 (1984), pages 53 to 58 to influence combustion operations by electric discharges, in particular corona discharges. In this case too, the flame stability is to be improved and the pollutant emissions reduced. Technical applications of the abovementioned effects are described in WO 96/01394 A1, U.S. Pat. No. 3,416,870 A and U.S. Pat. No. 4,111,636 A.

[0011] A common feature of all the methods which correspond to the prior art is that the electrodes which are required in order to generate the electric field or a discharge in the flame are arranged in such a manner that the flame is either located between the field-generating electrodes or is surrounded by one electrode. This electrode can be identical to the combustion chamber. An arrangement of this nature is illustrated with reference to FIG. 1 of the description. In any event, it is possible to draw a straight connecting line between electrodes of opposite polarity in such a manner that the connecting line passes through the flame which is to be influenced.

[0012] In FIG. 1, the direction of propagation of a flame 2 or the direction of flow of the exhaust gases is indicated as the z direction. The location z=0 is determined by the position at which the solid, liquid or gaseous fuel is transformed into the flame. No significant ionization caused by the combustion process occurs at locations z<0.

[0013] Arrangements which correspond to the prior art are without exception characterized by at least one electrode or one or more parts of such an electrode extending exclusively or predominantly over areas where z>0. In this case, it is also possible for the combustion chamber which surrounds the flame to be an electrode or part of an electrode. In extreme cases, the arrangement is such that partial areas of the flame may touch an electrode. In any event, it is possible to draw a straight connecting line from one electrode to an electrode of opposite polarity in such a way that the connecting line passes through the flame.

[0014] One drawback of the prior art described above is that the electric field which is generated by means of the electrodes passes through a large area of the flame, while the

[0015] actual effect of the electric field occurs in what is known as the flame front. The flame front is a narrow area, compared to the dimensions of the flame, between the cold fuel and the flame in which the chemical reactions leading to the formation of the flame take place. Since the flame has an electrical conductivity which is by no means negligible, on account of the charge carriers contained therein, the fact that the electric field passes through wide areas of the flame means that an electric current flows throughout the flame area which is enclosed by the electrodes, causing an increased energy consumption without contributing to the desired effect within the flame front. This is the case in particular if electrically conductive areas of the flame or its surroundings are in direct contact with the electrodes.

[0016] Working on this basis, it is an object of the invention to provide a method and to create the associated device which improve the influencing of combustion operations with fuels in a simple and economic way. The fuels to be used are in particular, although not exclusively, gases, preferably in premixed form.

[0017] According to the invention, the object is achieved by the measures described in patent claim 1. An associated device forms the subject matter of patent claim 4. Refinements to the method and/or device are given in the dependent claims.

[0018] In the invention, the flame is exposed to the action of an electric field, the field being configured in such a way that it passes through only those areas of the flame in which it has a stabilizing, pollutant-reducing effect. In the associated device, for this purpose electrodes are arranged and acted on by a voltage in such a way that an electric field preferably passes through those areas of the flame

[0019] in which it produces its stabilizing, pollutant-reducing effect. This is achieved by virtue of all the field-generating electrodes being arranged in areas in which no ionization or no significant ionization brought about by the combustion process occurs. This condition is satisfied if the electrodes are arranged on that side of the burner opening which is remote from the flame, in such a way that there is no straight connecting line between electrodes of opposite polarity which passes through the flame.

[0020] Particular advantages of the invention result if the system is assigned sensors and control devices which control the voltage applied to the electrodes in such a way that the combustion process is influenced in the desired way. There are advantageously sensors, one of which measures the frequency of any combustion oscillation which may be present and another of which measures the pollutant concentration in the exhaust gas. The sensors supply the input signal to a control unit which controls frequency, amplitude and phase of the voltage applied to the electrodes in such a way that the combustion oscillations and/or the pollutant concentration are minimized.

[0021] Further details and advantages of the invention will emerge from the following description of figures showing exemplary embodiments with reference to the drawing in conjunction with the patent claims. In the drawing:

[0022]FIG. 1 diagrammatically depicts an arrangement of the prior art, which has already been dealt with in the introduction,

[0023]FIGS. 2 and 3 diagrammatically depict two-different embodiments of the invention,

[0024]FIGS. 4 and 5 diagrammatically depict a plan view and sectional illustration of a further embodiment, and

[0025]FIG. 6 diagrammatically depicts a use of the invention for the combustion of solid materials.

[0026] In the individual exemplary embodiments, identical parts are provided with identical reference symbols. The embodiments are partially described jointly.

[0027] In all the examples, the burner is in each case denoted by 1 and the flame by 2. The burner 1 has a gas feed 3. Furthermore, the arrangement includes at least one electrode 6, which can be used to apply electric fields to the flame. The flame is scaled along the Z coordinate.

[0028]FIG. 1 has already been dealt with in the introduction, with corresponding statements relating to the prior art, to which reference is made in detail. The first exemplary embodiment of the invention, shown in FIG. 2, illustrates the flame 2 which is generated by the burner 1 for fuels which are in gas form, in liquid form or in the form of a solid powder transported in gas or liquids. The fuel is guided through the burner 1 into a combustion space 4 through the fuel inlet 3. The burner 1 may consist of electrically conductive or nonconductive material. In the former case, the burner serves as electrode, while in the latter case an electrode 5, which in this example is in the form of a ring and closely surrounds the burner 1, is fitted. A further electrode 6 is arranged in such a way as to lie completely in the area indicated by z≦0. The electrodes are electrically connected to the mains part 7.

[0029] In the exemplary embodiment shown in FIG. 3, a rod electrode 6 a is arranged coaxially inside the burner, in such a manner that it only projects into the area z<0 sufficiently far to satisfy the condition that there should be no straight connecting line between the electrodes. An electric field in the sense of the invention is formed between the electrode 6 a, on the one hand, and the burner 1, on the other hand, if the latter consists of electrically conductive material, or a further electrode 5, which in this specific case

[0030] surrounds the burner, which is assumed to be nonconductive, in a form-fitting manner. In a modification of the invention, the rod electrode 6 a located inside the burner can be replaced by a tube or nozzle or a plurality of tubes and nozzles which are by combustible or noncombustible gases or gas mixtures. On account of the short electrode-to-electrode distance compared to the prior art shown in FIG. 1, it is already possible to generate an electric field with correspondingly lower voltages.

[0031] The use of the invention is not restricted to the combustion of liquid or gaseous fuels. FIG. 6 indicates a combustion chamber 8 in which a pile 11 of solid fuel, for example coal, is burning on a grate 10 which is electrically insulated from the combustion chamber by nonconductive support elements 9. The plane z=0 is defined by the top edge of the grate 10 or, if the fuel is electrically conductive, by the upper limit of the pile 11. An annular electrode 12 is arranged in such a way that it projects into the area z<0 at most sufficiently far to satisfy the condition that there must be no straight connecting line between the electrodes which passes through the flame.

[0032] The invention is not restricted either to systems which comprise just two electrodes, one of which may be the burner, or to electrodes which are rotationally symmetrical with respect to the burner axis, i.e. in particular annular, toroidal or cylindrical. FIGS. 4 and 5 show an exemplary embodiment in which a plurality of rod electrodes 13 a, 13 b, 13 c, 13 d are arranged radially with respect to a burner 1.

[0033] In the arrangements described with reference to FIGS. 2 to 6, the desired influencing of the combustion process by electric fields is achieved without the electric field passing through extensive parts of the flame in which it does not have a stabilizing or pollutant-reducing effect. Contact with the electrodes by electrically conductive

[0034] areas of the flame is as far as possible avoided. As a result, the current induced by the electric field is considerably reduced, and the demand for electric power is reduced to the same degree. Furthermore, the likelihood of disruptive electrical sparkovers greatly decreases. The relatively short electrode-to-electrode distance leads to a reduced voltage consumption compared to the prior art for the same electric field strength.

[0035] The arrangements according to the invention for influencing flames with the aid of electrical means are equally suitable for operation with DC voltage, pulsed or clocked DC voltage and AC voltage, as well as DC voltage with superimposed AC voltage. If a DC voltage is used, the polarity of the burner is preferably negative.

[0036] Furthermore, sensors can be assigned to the system: a first sensor records the frequency of any combustion oscillations which may be present. A second sensor measures the pollutant concentration in the exhaust-gas stream from the flame. The sensors supply input signals for a control unit (not shown in detail) which controls the frequency, amplitude and phase of the voltage applied to the electrodes in such a manner that the combustion oscillations and the pollutant concentration are minimized. 

1. A method for influencing combustion operations with fuels, in which electrical means are used to guide and/or alter a flame, comprising the following measures: the flame is exposed to the action of an electric field, the field-generating electrodes are arranged on that side of the burner opening which is remote from the flame, in such a manner that there is no straight connecting line between electrodes of opposite polarity which passes through the flame, with the result that the electric field passes through only those areas of the flame in which it has a stabilizing, pollutant-reducing effect.
 2. The method as claimed in claim 1, characterized in that premixed gases are used.
 3. The method as claimed in one of the preceding claims, characterized in that thermo-acoustic emissions are reduced.
 4. A device for carrying out the method as claimed in claim 1 or one of claims 2 or 3, using stabilizing, pollutant-reducing means to influence the flame during a combustion operation, the means being formed by electrodes at the burner, characterized in that the electrodes (5, 6) are arranged outside the area of the flame (2) and are arranged that side of the burner opening which is remote from the flame, and there is no straight connecting line between electrodes of opposite polarity which passes through the flame.
 5. The device as claimed in claim 4, characterized in that there is at least one annular electrode (6) which surrounds the burner (1) over a wide area.
 6. The device as claimed in claim 5, characterized in that there is also an annular electrode (5) directly on the burner (1) on that side of the burner opening which is remote from the flame.
 7. The device as claimed in one of claims 4 to 6, characterized in that a rod electrode (6 a) is arranged coaxially inside the burner (1).
 8. The device as claimed in one of claims 4 to 7, characterized in that the burner (1) is surrounded by a plurality of electrodes (12, 13) arranged on the circumference around the flame (2).
 9. The device as claimed in claim 8, characterized in that the electrodes (12, 13) are arranged symmetrically around the burner (1).
 10. The device as claimed in one of claims 11 or 12, characterized in that the electrodes are rod electrodes (13 a to 13 d) directed centrally at the burner (1).
 11. The device as claimed in one of claims 4 to 10, characterized in that there are sensors for recording the frequency and amplitude of combustion oscillations and/or the pollutant concentration in the exhaust-gas stream, these sensors, by means of at least one control device, controlling frequency, amplitude and phase of the applied voltage in such a way that the combustion oscillations and/or pollutant concentration in the exhaust gas are minimized. 